Python 2.8 - eliminating shadow by adding emission shader to background plane/image
I suggest that you learn how to do this manually first.

To create any material you will need a material output and link the emissive shader to that.
Use the Shader Editor to review your result.

import bpy

bpy.ops.object.select_all(action='DESELECT')
bpy.data.objects['Sky'].select_set(True)

obj = bpy.data.objects['Sky']

skymaterial = bpy.data.materials.new('SkyMat')
obj.active_material = skymaterial  #UNWANTED RESULTS START HERE
skymaterial.use_nodes = True
nodes = skymaterial.node_tree.nodes
nodes.clear()


# Create needed Nodes
nodeOut = nodes.new(type='ShaderNodeOutputMaterial')
nodeEmission = nodes.new(type='ShaderNodeEmission')
nodeTexture = nodes.new(type='ShaderNodeTexImage')


# Link them together
links = skymaterial.node_tree.links
linkOut = links.new(nodeEmission.outputs[0], nodeOut.inputs[0])
linkTexture = links.new(nodeTexture.outputs[0], nodeEmission.inputs[0])

# Configure Texture node
img = bpy.data.images.load('//test.png')
img.name = 'Test'
nodeTexture.image = img